The invention is generally related to the installation of topsides of offshore structures and more particularly to the leg mating units between the topsides and the supporting legs.
Traditionally, the standard methodology for installing a topside structure onto its supporting structure, whether the supporting structure is bottom founded or a floating structure, involves lowering the topside structure onto the lower supporting structure.
The topside structure may be supported on a barge or pontoons used to position the topside structure over the lower supporting structure and legs. Once in position, the barge or pontoons are ballasted down to transfer the weight of the topside to its supporting structure.
The topside may also be lifted from the barge by a crane and lowered into position on the legs of the supporting structure by the crane.
In either method of installation, the weight transfer of the topside to the supporting structure must be controlled to prevent damage to both the topside and the supporting structure. Also, the heave motion of waves acting on the barge, pontoons, or vessel on which the crane is mounted must be taken into account to prevent damage to the topside and its lower support structure by repeated contact due to wave action. In order to eliminate or greatly reduce the chances for damage to the topside and legs of the lower support structure, it is preferable to quickly transfer a predetermined portion of the load of the topside onto the legs of the lower support structure. This serves to retain contact between the two structures and prevent damage that would occur due to wave action causing repeated contact. Because topsides structures can weigh as much as 80,000 tons, the full load cannot be immediately transferred. Otherwise, significant damage would occur to the supporting structure. After the initial transfer to insure that contact is maintained, the transfer of the remaining weight is done in a controlled manner.
This has typically been addressed in the offshore construction industry by the use of sacrificial leg mating units (LMUs).
The sacrificial LMUs have been mounted on the inside of the legs of the lower support structure and been a crushable material such as elastomeric material that is designed to absorb the initial load and then progressively crush as the remainder of the topside load is transferred. As a result of the size and weight of the structures involved, and the forces that must be dealt with, a single LMU can cost as much as one million dollars or more, and larger offshore structures can require as many as twelve LMUs during installation. Because the load absorbing characteristic of the LMUs is destroyed, and the LMUs have been an integral member of the legs, they are not reusable.
It is seen from the above that a more cost and material efficient means of achieving the same result is desirable in the offshore construction and installation industry.